1. Field of the Invention
The present invention is concerned with method and apparatus for measuring movement, displacement and/or deformation of the surface of a rotating and/or torque transmitting element. More particularly the invention is concerned with non-contacting measurement of the speed and/or torque of a rotating element.
2. Discussion of Prior Art
Engineers and scientists have long wished to produce simple, reliable and accurate means for measuring the torque in and/or speed of elements such as rotating shafts. Measurements of torque and/or speed are necessary for diagnosis, prognosis and load level monitoring of rotary drive systems such as plane, car and ship engines, motors and generators, rotating machine tools etc.
The measurement of the mechanical power produced by an engine requires one to know both the torque and rotational speed of a shaft. The accurate on-line measurement of speed and torque is therefore critical for the determination of on-line power and efficiency of rotary drive devices.
Speed Measurement
Rotation speed is measured by detecting a periodic signal induced in a sensor by one or more markers or elements on the surface of the rotating element. The sensor or sensors are typically located near the surface of the rotary element (e.g. drum, disc, shaft or similar) and a signal is generated when a pre-determined and/or marked portion or portions of the element pass near the sensor. The rotation speed can be determined by measuring the time between the successive passages of the pre-determined marked portion or portions of the rotary device past the sensor or sensors.
There are a number of methods of creating and then noting the passage of surface markings or discontinuities required by the known speed measurement methods. These include:
a) Method of Variable Magnetic Reluctance
This involves the use of a coil and a permanent magnet located near a gear wheel which detects the disturbance of the magnetic flux produced by the permanent magnet as the teeth of the gear wheel pass through the magnetic field generated by the coil. An example of such a system is described in U.S. Pat. No. 3,980,913.
b) Magnetoresistive
Alternatively, magnetic markers are attached to the rotating element and the resulting periodic variation of magnetic flux can be detected using magnetoresistance sensors as shown in U.S. Pat. No. 5,754,042, or in K Miyashita et al. “Non-contact Magnetic torque sensor”, IEEE Trans. Mgn., Vol. 26 No. 5, p. 1560 (1990).
c) Optical Methods
Rotation speed can also be detected optically by monitoring the reflection of light off the surface of a rotating element which contains marked regions of different optical reflectivity as shown in U.S. Pat. No. 4,639,595.
All of the known methods described above require some modification of the surface of the rotary element whose speed is being measured. A discontinuity or marker must be placed on the rotary element before its speed can be measured. These known systems are therefore relatively expensive and difficult to fit, particularly to fit to existing machinery or engines.
Other non-contact methods for measuring torque, which do not require modification of the rotating element, include those described in GB 2008765, GB 1586080, EP 0168692, EP 0167656, EP 0103354 and EP 0046517.
All these systems require that the rotating elements are made from a magnetic material and exploit the principle of magnetostriction. Magnetostriction refers to an interaction between magnetic and elastic properties of a material.
Application of a torque to a rotating element will result in the creation of a stress field in the element and a corresponding change in bulk magnetic permeability. An alternating magnetic field is applied to the element, so that it penetrates the element, and changes in the field caused by changes in bulk magnetic permeability are measured by a detector. It therefore can be determined with knowledge of the physical properties of the element, what torque is being applied to the rotating element.
These systems ignore the naturally occurring variations of magnetic properties in the material of a rotating object, treating it as noise, whilst measuring the changes in magnetic properties of the object with the applied stress. Embodiments of the present invention, however, make use of the naturally occurring variations and uses the cyclical repetition of a pattern past a sensor as the means for measuring rotation speed and torque.
Torque Measurement
Measurement of the torque transmitted by rotating elements is usually based on either the angular deflection or twist of a dedicated section of the torque transmitting element or from the strain at the surface of the torque transmitting element.
In one known method, twist of a rotating shaft or torque transmitting element is measured by generating a periodic signal from the monitoring of the movement or displacement of markings placed around the circumference of the shaft. Two sets of markings are used with one set at either end of a dedicated section whose twist is to be measured. The relative movement of the two separated sets of markings is a function of the twist of the section between the markings. U.S. Pat. No. 4,602,515 shows such a system in which torque is determined from the phase shift between the two periodic signals generated magnetically.
In another method, twist is measured by fixing a tubular structure surrounding the torque transmitting element at one end of the dedicated section whose twist is to be measured and keeping the tubular structure loose at the other end. Torque can then be determined from the relative concentrical displacement between the torque transmitting element and the tubular structure at the loose end using optical means as shown in U.S. Pat. No. 5,606,137, inductive means as shown in Krimmell W., “Induktiver Aufnehmer mit Zukunft”, Messen, Prüfen, Automatisieren, p. 614 (1985) or capacitive means as shown in Wolffenbüttel, Non-contact capacitive torque sensor for use on a rotating code IEEE Trans. Instrum. Measure, vol. 39 no 6 p. 1008(1990).
For a circular shaft of length L, relative twist θ between its ends, shear modulus G and moment of inertia J,
  T  =            G      ⁢                          ⁢      J      ⁢                          ⁢      θ        L  
Consequently for a shaft of constant G, J and L, torque is directly proportional to twist and easily determined therefrom provided G, J and L are known.
As with the methods of speed measurement previously described, the known methods of torque measurement require the surface of a rotating torque transmitting element such as a disc, drum or shaft to be modified to have the necessary markings and/or discontinuities.